During the past decade we have witnessed dramatic changes in the nature of the terrorist threat. In the 1990s, right-wing extremism overtook left-wing terrorism as the most dangerous domestic terrorist threat to the country.
Both are dwarfed by the Islamic terrorism of course.
That said, when he wrote this in 2002 he was probably right. You had right wing extremists like McVeigh, and less active left wing extremist scene compared with 1960s-80s.
But now things are different. Widespread rioting and unrests (called "uprisings" by those engaging in it) since 2014, by Antifa and #BLM which has Marxist roots and is glorifying 70s terrorists like Joanne Chesimard. You have resurgence of hate groups like Nation of Islam, New Black Panther Party, NFAC (who hid a cop killer in their compound in my neck of the woods recently), Black Hammer Party etc.
Feds investigating Black Hammer Party in wide-ranging criminal probe
It is not politically correct to point to those groups, as the left wing terrorist groups are often "minority led", but they are a real threat.